Aliphatic polyurethanes containing acryloyl groups and a process for their preparation

ABSTRACT

The present invention relates to aliphatic polyurethanes which contain acryloyl groups and are based on the reaction product (at an NCO/OH equivalent ratio of 0.5:1 to 1:1) of 
     a) 1.0 mole of a diisocyanate component containing at least 50 equivalent percent, based on NCO equivalents, of hexamethylene diisocyanate with 
     b) 0.5 to 0.85 moles of at least one monohydric alcohol having a molecular weight of 116 to 144 and containing acryloyl groups. 
     c) 0.15 to 0.5 moles of a monohydric alcohol having a molecular weight of 154 to 2000 and containing ester groups or a mixture of alcohols having an average molecular weight of 154 to 2000 and an average hydroxyl functionality of 0.8 to 1.5 and containing ester groups and 
     d) 0.3 to 0.5 moles of at least one polyhydric alcohol having a molecular weight of 62 to 2000. 
     The present invention also relates to a process for the preparation of these polyurethanes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to certain polyurethanes which containacryloyl groups and can be converted into crosslinked products byradiation curing and to a process for their preparation.

2. Description of the Prior Art

Polyurethanes which contain acryloyl groups and are known and may beobtained, as described, for example, in DE-OS (German PublishedSpecification) 1,644,798, DE-OS 2,115,373 or DE-OS 2,737,406, as thereaction product of, inter alia, polyisocyanates and hydroxyalkylacrylates. They are cured with the aid of high-energy rays, such as UVrays or electron beams, and are used for coating, for example, wood orfor the production of coatings for graphical purposes.

A wide variety of polyisocyanates can be employed for the preparation ofthe urethane acrylates. If the production of surface coating films whichare resistant to yellowing is desired, (cyclo)aliphatic polyisocyanatesare employed for the preparation of the urethane acrylates. Althoughhexamethylene diisocyanate, inter alia, is mentioned in DE-OS (GermanPublished Specification) 2,115,373 as a suitable starting diisocyanate,it can be said that this is a purely hypothetical disclosure, sinceprior to the present invention it has not been possible to employ thisinexpensive diisocyanate in practice because of the crystallinity of theurethane acrylates formed, which prohibits their use as surface coatingbinders.

Therefore, an object of the present invention is to provide aliphaticpolyurethanes containing acryloyl groups which are prepared fromhexamethylene diisocyanate (HDI) as the essential starting diisocyanateand which nevertheless are liquid and crosslink rapidly under UVirradiation.

This object has been achieved by the aliphatic polyurethanes containingacryloyl groups according to the invention which are described in moredetail below.

SUMMARY OF THE INVENTION

The present invention relates to aliphatic polyurethanes which containacryloyl groups and are based on the reaction product (at an NCO/OHequivalent ratio of 0.5:1 to 1:1) of

a) 1.0 mole of a diisocyanate component containing at least 50equivalent percent, based on NCO equivalents, of hexamethylenediisocyanate with

b) 0.5 to 0.85 moles of at least one monohydric alcohol having amolecular weight of 116 to 144 and containing acryloyl groups,

c) 0.15 to 0 5 moles of a monohydric alcohol having a molecular weightof 154 to 2000 and containing ester groups or a mixture of alcoholshaving an average molecular weight of 154 to 2000 and an averagehydroxyl functionality of 0.8 to 1.5 and containing ester groups and

d) 0.3 to 0.5 moles of at least one polyhydric alcohol having amolecular weight of 62 to 2000.

The present invention also relates to a process for the preparation ofaliphatic polyurethanes containing acryloyl groups by reacting at anNCO/OH equivalent ratio of 0.5:1 to 1:1

a) 1.0 mole of a diisocyanate component containing at least 50equivalent percent, based on NCO equivalents, of hexamethylenediisocyanate with

b) 0.5 to 0.85 moles of at least one monohydric alcohol having amolecular weight of 116 to 144 and containing acryloyl groups,

c) 0.15 to 0.5 moles of a monohydric alcohol having a molecular weightof 176 to 2000 and containing ester groups or a mixture of alcoholshaving an average molecular weight of 154 to 2000 and an averagehydroxyl functionality of 0.8 to 1.5 and containing ester groups and

d) 0.3 to 0.5 moles of at least one polyhydric alcohol having amolecular weight of 62 to 2000.

DETAILED DESCRIPTION OF THE INVENTION

Polyisocyanate component a) contains at least 50 equivalent percent,based on NCO equivalents, of HDI. Polyisocyanate component a) can alsocontain up to 50% NCO equivalent percent of other polyisocyanatescontaining aliphatically and/or cycloaliphatically bonded isocyanategroups. These other polyisocyanates include1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (IPDI),polyisocyanates based on IPDI or HDI which contain isocyanurate groupsand polyisocyanates based on IPDI or HDI and polyhydric alcohols (suchas trimethylolpropane and/or propylene glycol) and contain urethanegroups. Polyisocyanate component a) preferably contains at least 80 NCOequivalent percent, more preferably 100 NCO equivalent percent, ofhexamethylene diisocyanate (HDI).

Component b) is based on a monohydric alcohol containing acryloyl groupsand mixtures of these alcohols. These are understood to be esterscontaining on average one free hydroxyl group per molecule and preparedby reacting acrylic acid with dihydric alcohols. The esters have amolecular weight of 116 to 144. Examples of such ethylenicallyunsaturated esters include hydroxyethyl acrylate,1-methyl-2-hydroxyethylacrylate, 2- and 3-hydroxypropyl acrylate and 2-,3- and 4-hydroxybutyl acrylate.

Component c) is based on a monohydric alcohol containing ester groups ora mixture of alcohols containing ester groups and having an averagehydroxyl functionality of 0.8 to 1.5 and an average molecular weight of154 to 2000, preferably 176 to 1000. Component c) can containolefinically unsaturated double bonds (for example because of the use ofacrylic acid as a component for the preparation of the ester alcohols),but can also be exclusively prepared from from saturated compounds.Examples of suitable compounds c) include ester mixtures having anaverage hydroxyl functionality of 0.8 to 1.5, which can be obtained, forexample, by the reaction of acetic acid, acetic anhydride or acrylicacid with polyhydric alcohols at an equivalent ratio of carboxyl groupsto hydroxyl groups of 1.5:3 to 2.2:3, preferably 2:3. An equivalentamount of an acid anhydride may be used. Examples of polyhydric alcoholsinclude trimethylolpropane, glycerol and the alkoxylation products ofpolyhydric alcohols (e.g., the addition product of 4 moles of ethyleneoxide to 1 mole of trimethylolpropane).

The polyols d) include polyhydric alcohols having a molecular weight of62 to 2000, preferably alcohols having a molecular weight of 92 to 200which are at least trihydric such as glycerol, trimethylolpropane,pentaerythritol and sorbitol. Also suitable are polyether and polyesterpolyols having molecular weights of up to 2000, preferably up to 1000.The polyether polyols may be obtained by the alkoxylation of monomericalcohols of the type previously mentioned. The polyester polyols may beobtained by the reaction of polyhydric alcohols and polybasic acids oranhydrides of polybasic acids in known manner. It is also possible touse relatively small amounts of diols, such as ethylene glycol,thiodiglycol or hexanediol. However, the diols should be used in amountsof less than 50 hydroxyl equivalent percent, based on the hydroxylequivalents of component d). Otherwise, it is not possible to obtain thedesired properties of the polyurethanes.

The polyurethanes can be prepared by reaction of the starting componentsin the presence or absence of solvents which are inert towardsisocyanates such as acetone, ethyl acetate, butyl acetate, low molecularweight esters of (meth)acrylic acids or mixtures of such solvents. Thereaction temperature is preferably 20° to 100° C., more preferably 40°to 80° C. The reaction is preferably carried out as a batch process withthe optional addition of suitable catalysts. The reaction temperature ismaintained until no further free isocyanate groups are present. Itaccordance with one embodiment polyisocyanate component a) is initiallyintroduced into the reaction vessel, components b) and c) are addedtogether or in succession (the sequence is not critical), and componentd) is then added. In accordance with another embodiment components b),c) and d) are initially introduced into the reaction vessel and thepolyisocyanate component a) is subsequently added.

In all cases, the reaction is carried out under mild conditions at anNCO/OH equivalent ratio of 0.5:1 to 1:1 until the NCO content has fallento below 0.1% by weight.

The addition reaction resulting in the polyurethane can be acceleratedin known manner by the addition of a suitable catalyst, e.g., tinoctoate, dibutyltin dilaurate or tertiary amines such asdimethylbenzylamine. Alternatively, the urethane acrylate obtained asthe reaction product can be protected from premature and undesirablepolymerization by the addition of suitable inhibitors and antioxidantssuch as phenols and/or hydroquinones, in amounts of in each case 0.001to 0.3% by weight, based on the urethane acrylate. These auxiliaries canbe added before, during and/or after the reaction.

The surface coatings prepared from the products according to theinvention can contain the known auxiliaries and additives of surfacecoating technology.

These include fillers, pigments, dyestuffs, thixotropic agents, glossingagents, matting agents and flow control agents. These additives andauxiliaries can be used in the customary amounts. The polyurethanescontaining acryloyl groups can also be dissolved in known surfacecoating solvents, mixed with copolymerizable monomers or employed as anemulsion in water.

Examples of suitable solvents include butyl acetate, cyclohexane,acetone, toluene or mixtures of such solvents.

Examples of suitable copolymerizable monomers are organic compoundswhich contain at least one copolymerizable olefinic double bond permolecule and have a viscosity of not more than 500 mPa.s at 23° C.Suitable monomers include styrene, 1,6-hexanediol diacrylate,trimethylolpropane triacrylate or N-vinylpyrrolidone.

The inert solvents and also the copolymerizable monomers can be employedin amounts in each case of up to 100% by weight, preferably up to 50% byweight, based on the weight of the polyurethanes containing acryloylgroups.

It is also possible to simultaneously use both inert solvents andcopolymerizable monomer.

If the polyurethanes containing acryloyl groups are to be processed froman aqueous emulsion, the emulsion can be prepared, for example, with theaid of external emulsifiers and if appropriate the customary auxiliariesused in emulsion technology.

Emulsifiers which are suitable for this purpose are known and described,for example, in Ullmanns Encyclopadie der tech. Chemie (UllmannsEncyclopaedia of Industrial Chemistry), Volume 10, 4th Edition, chapteron Emulsions, page 449 et seq.

Preferred emulsifiers are copolymerizable polyurethane emulsifierscontaining acryloyl groups, such as those described in DE-OS (GermanPublished Specification) 3,900,257.

The aqueous emulsions contain 10 to 70% by weight, preferably 30 to 70%by weight, of the polyurethanes containing acryloyl groups according tothe invention. The emulsions can be prepared by stirring water into themixture of polyurethane acrylate according to the invention andemulsifier, for example by simple stirring or by means of a dissolver.

To form a finely divided emulsion, it is advantageous to add the waterin portions at temperatures below 30° C. Oil-in-water emulsions areformed under optimum shear.

In all the variants of the process according to the invention, thecuring (if appropriate after evaporation of volatile auxiliaries, suchas inert solvents or water) is carried out by means of high-energyradiation, such as UV light, electron beams or gamma rays.

In the case of crosslinking by means of UV irradiation, it is necessaryto add photoinitiators to the coating compositions. Suitablephotoinitiators are known and include the compounds described, forexample, in the monograph by J. Korsar, Light-Sensitive Systems, J.Wiley & Sons, New-York - London - Sydney 1965.

Preferred photoinitiators are benzoin ethers such as benzoin isopropylether, benzil ketals such as benzil dimethyl ketal, andhydroxyalkylphenones such as 2-hydroxy-2-methylphenylpropan-1-one.

The photoinitiators are employed in amounts of 0.1 to 10% by weight,preferably 0.1 to 5% by weight, based on the weight of the reactionproducts, depending on the intended purpose of the composition accordingto the invention. The photoinitiators can be used individually or incombination with one another to obtain advantageous synergistic effects.

The coating compositions or surface coatings prepared using thepolyurethanes according to the invention as binders are suitable forcoating diverse substrates such as paper, cardboard, leather, wood,plastics, non-wovens, textiles, ceramic materials, mineral materials,glass, metal, imitation leather and photographic materials (for example,paper provided with a photographic layer).

In the following examples, all the percentages and amounts are by weightunless otherwise indicated.

EXAMPLES

The amount of component a) set forth in Table 1 was dissolved, togetherwith 0.2% of tin dioctoate and 0.1% of 2,6-di-t-butyl-p-cresol, in butylacetate (in an amount to provide a solids content of 80%) and thesolution was heated to 50° C. while air was passed through. The amountof component b) set forth in Table 1 was then added dropwise, followedby the amount of component c) and finally the amount of component d).During this procedure, the reaction temperature was kept at a maximum of60° C., if necessary by external cooling. The mixture was then kept at60° C. until the NCO content had fallen to less than 0.1%

                  TABLE 1                                                         ______________________________________                                                      1    2      3      4     5                                      ______________________________________                                                      Amount weighed out (moles)                                      Component a)                                                                  Hexamethylene diisocyanate                                                                    1.0    1.0    1.0  1.0                                        Isophorone diisocyanate                  1.0                                  Component b)                                                                  Hydroxyethyl acrylate                                                                         0.8    0.8    0.8  0.9   0.8                                  Component c)                                                                  Trimethylolpropane                                                                            0.2    0.2                                                    diacetate ethoxylated 4-fold                                                  Trimethylolpropane            0.2  0.1   0.2                                  diacrylate ethoxylated 4-fold                                                 Component d)                                                                  Trimethylolpropane                                                                            0.33          0.33 0.33  0.33                                 Trimethylolpropane     0.33                                                   ethoxylated 4-fold                                                            Viscosity (23° C., mPa · s)                                                   6500   4500   9950 crystal-                                                                            78400                                                                   line                                       ______________________________________                                    

Use examples

The product from Comparison Example 4 was not suitable for use as acoating because of its crystallinity. For clarity, the numbering of theuse examples corresponds to the previous numbering of the preparationexamples.

The products of the examples according to the invention and ComparisonExample 5 were adjusted with butyl acetate to a processing viscosity ofabout 50 seconds (DIN cup 4 mm, 23° C.), and in each case 5% ofbenzophenone, based on the weight of the urethane acrylate, was added.After the surface coating films had been applied to cardboard(application thickness of 10 μm), the coated substrate was passedthrough the beam of a Hanovia lamp (80 W/cm, 10 cm distance). The valuegiven for the reactivity in Table 2 indicates the belt speed at which acoating which was resistant to solvents and scratching was achieved.

"Resistant to solvents" in this case means that the surface coating filmstill looked perfect after at least 20 double rubs with a cloth soakedin butyl acetate.

                  TABLE 2                                                         ______________________________________                                                              Comparison                                                        Example     Example                                                           1   2         3     5                                               ______________________________________                                        Reactivity  40    55        60  <10                                           (m/minute)                                                                    ______________________________________                                    

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. An aliphatic polyurethane which contains acryloylgroups and comprises the reaction product (at an NCO/OH equivalent ratioof 0.5:1 to 1:1) ofa) 1.0 mole of a diisocyanate component containing atleast 50 equivalent percent, based on NCO equivalents, of hexamethylenediisocyanate with b) 0.5 to 0.85 moles of at least one monohydricalcohol having a molecular weight of 116 to 144 and containing acryloylgroups, c) 0.15 to 0.5 moles of a monohydric alcohol having a molecularweight of 154 to 2000 and containing ester groups or a mixture ofalcohols having an average molecular weight of 154 to 2000 and anaverage hydroxyl functionality of 0.8 to 1.5 and containing ester groupsand d) 0.3 to 0.5 moles of at least one polyhydric alcohol having amolecular weight of 62 to
 2000. 2. A process for the preparation of analiphatic polyurethane containing acryloyl groups which comprisesreacting at an NCO/OH equivalent ratio of 0.5:1 to 1:1a) 1.0 mole of adiisocyanate component containing at least 50 equivalent percent, basedon NCO equivalents, of hexamethylene diisocyanate with b) 0.5 to 0.85moles of at least one monohydric alcohol having a molecular weight of116 to 144 and containing acryloyl groups, c) 0.15 to 0.5 moles of amonohydric alcohol having a molecular weight of 176 to 2000 andcontaining ester groups or a mixture of alcohols having an averagemolecular weight of 154 to 2000 and an average hydroxyl functionality of0.8 to 1.5 and containing ester groups and d) 0.3 to 0.5 moles of atleast one polyhydric alcohol having a molecular weight of 62 to 2000.